Introduce few const getters for dom classes

third_party/WebKit/Source/core/events/EventTarget.h

Index: third_party/WebKit/Source/core/events/EventTarget.h
diff --git a/third_party/WebKit/Source/core/events/EventTarget.h b/third_party/WebKit/Source/core/events/EventTarget.h
index 7c701055c5f7f652501625819f21596eb45a5292..2b9c446cc25a3bd9285b7de29fc651deed44b8c0 100644
--- a/third_party/WebKit/Source/core/events/EventTarget.h
+++ b/third_party/WebKit/Source/core/events/EventTarget.h
@@ -172,7 +172,7 @@ protected:
     virtual DispatchEventResult dispatchEventInternal(Event*);
 
     // Subclasses should likely not override these themselves; instead, they should subclass EventTargetWithInlineData.
-    virtual EventTargetData* eventTargetData() = 0;
+    virtual EventTargetData* eventTargetData() const = 0;
     virtual EventTargetData& ensureEventTargetData() = 0;
 
 private:
@@ -204,7 +204,11 @@ public:
     }
 
 protected:
-    EventTargetData* eventTargetData() final { return &m_eventTargetData; }
+    EventTargetData* eventTargetData() const final
+    {
+        return const_cast<EventTargetData*>(&m_eventTargetData);
+    }
+
     EventTargetData& ensureEventTargetData() final { return m_eventTargetData; }
 
 private: